<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<meta http-equiv="X-UA-Compatible" content="IE=edge">
	<meta name="viewport" content="width=device-width, initial-scale=1.0">
	<title>Document</title>
	<style>
		/* 两侧定宽  中间自适应 */
		.left,.right{
			width: 300px;
			height: 200px;
			background-color: red;
		}
		.left{
			float: left;
		}
		.right{
			float: right;
		}
		.center{
			height: 500px;
			background-color: blue;
			/* 开启BFC */
			overflow: auto;
		}
	</style>
</head>
<body>
	<div class="left"></div>
	<div class="right"></div>
	<div class="center"></div>
</body>
</html>